The present invention relates to communications equipment, and, more particularly, to blind equalization in a receiver.
In blind equalization, the adaptive filters of a receiver are converged without the use of a training signal. As known in the art, there are two techniques for blind equalization: one is referred to herein as the xe2x80x9creduced constellation algorithrmxe2x80x9d (RCA) (e.g., see Y. Sato, xe2x80x9cA Method of Self-Recovering Equalization for Multilevel Amplitude-Modulation Systems,xe2x80x9d IEEE Trans. Commun., pp. 679-682, June 1975; and U.S. Pat. No. 4,227,152, issued Oct. 7, 1980 to Godard); and the other technique is the so-called xe2x80x9cconstant modulus algorithmxe2x80x9d (CMA) (e.g., see D. N. Godard, xe2x80x9cSelf-Recovering Equalization and Carrier Tracking in Two-Dimensional Data Communications Systems,xe2x80x9d IEEE Trans. Commun., vol. 28, no. 11, pp. 1867-1875, November 1980; and N. K. Jablon, xe2x80x9cJoint Blind Equalization, Carrier Recovery, and Timing Recovery for High-Order QAM Signal Constellationsxe2x80x9d, IEEE Trans. Signal Processing, vol. 40, no. 6, pp. 1383-1398, 1992.) Further, U.S. Pat. No. 5,793,807, issued Aug. 11, 1998 to Werner et al., and entitled xe2x80x9cblind Equalization,xe2x80x9d presents a new blind equalization techniquexe2x80x94the multimodulus algorithm (MMA)xe2x80x94as an alternative to the above-mentioned RCA and CMA approaches.
However, for all blind equalization approaches the most fundamental performance issue is the ability to achieve reliable initial convergencexe2x80x94else the adaptive filter may converge to a wrong solution such as the well-known xe2x80x9cdiagonal solution.xe2x80x9d
Generally speaking, the RCA algorithm has less reliable convergence than either the CMA or MMA algorithms. As between the CMA and MMA algorithms, these algorithms have both benefits and drawbacks. For example, the CMA algorithm provides more reliable convergencexe2x80x94thus avoiding incorrect diagonal solutionsxe2x80x94but the CMA algorithm requires an expensive rotator. In comparison, the MMA algorithm does not require an expensive rotator but is more susceptible than the CMA algorithm to incorrect convergence.
However, there are some alternatives for reducing the occurrence of diagonal solutions. U.S. Pat. No. 5,835,731, issued Nov. 10, 1998 to Werner et al., presents a blind equalization algorithm referred to as the constrained Hilbert cost function (CHCF). The CHCF algorithm uses the Hilbert transfer function and dot-product properties of the in-phase and quadrature filters to prevent the occurrence of diagonal solutions. U.S. Pat. No. 5,809,074, issued Sep. 15, 1998 to Werner et al., presents a blind equalization technique referred to as the transition algorithm. In the latter, generally speaking, an adaptive filter first uses the CMA algorithm and then switches to using the MMA algorithm. Finally, the co-pending U.S. Patent application of Werner et al., entitled xe2x80x9cBlind Equalization Algorithm with Joint Use of the Constant Modulus Algorithm and the MultiModulus Algorithmxe2x80x9d Ser. No. 09/066189, filed on Apr. 24, 1998, presents a method for performing blind equalization using a joint CMA-MMA blind equalization algorithm.
We have discovered another technique for use in blind equalization of an adaptive equalizer that reduces the rate of occurrence of a diagonal solution. In particular, and in accordance with the invention, a receiver performs blind equalization as a function of both the constant R and the sliced symbols Ân.
In an embodiment of the invention, a receiver comprises an adaptive filter having a two-filter structure. The receiver uses a modified form of the CMA algorithm to perform blind equalization. This modified form of CMA is referred to herein as symbol CMA (SCMA). SCMA uses both the constant R and the sliced symbols, Ân, in performing blind equalization. Advantageously, SCMA can rotate a constellation, whereas CMA cannot.